The present invention relates to a device for purifying exhaust gas of diesel engines for removing particulate matters from the exhaust gas.
Regulations against the exhaust gases of diesel engines mounted on vehicles are becoming stringent year after year. In particular, it is becoming an urgent necessity to decrease the particulate matters (hereinafter abbreviated as PMs) which consists of carbon as a main component. As a device for removing the PMs from the exhaust gas, there has been known a diesel particulate filter (hereinafter abbreviated as DPF), and a trend toward obligatorily furnishing the vehicles mounting diesel engines with the DPF becomes also serious.
The DPF with which the diesel engine mounted on the vehicle is furnished must be regenerated by burning the PMs that have been trapped because the PMs deposit on the DPF due to repetitive operation of the engine. As a means for regeneration of the DPF, there is a method to burn the PMs by heating them with an electric heater, a burner or the like. According to this method, a system that a plurality of DPFs are arranged in parallel in the exhaust gas passage to alternately conduct the trapping and the burning is constituted. As another means for regenerating the DPF, there has also been studied a so-called continuous regeneration type DPF according to which an oxidizing catalyst is disposed in the exhaust gas passage on the upstream side of the DPF, NO in the exhaust gas is oxidized into NO2 by the oxidizing catalyst, and the PMs are continuously burned with NO2 while trapping the PMs as disclosed in, for example, Japanese Patent No. 3012249. Further, as another continuous regeneration type DPF, there has been known a method in which a NOx occluding/reducing catalyst is carried on the DPF and the PMs trapped are continuously burned by using active oxygen that generates when the NOx is occluded and reduced, as disclosed in Japanese Patent No. 2600492. In both of these continuous regeneration type DPFs, the PMs burn in a low temperature region of 250 to 400xc2x0 C. (which may shift up and down to some extent depending on the material of the catalyst) without requiring any particular heating means such as electric heater, burner or the like, giving such an advantage that the devices as a whole can be constructed simply and in a compact size.
Although it has been thus made possible to burn the PMs in a range of a so-called active temperature region of the catalyst, which can be easily accomplished by the exhaust gas temperature of the engine as described above, there may occur an operation condition that does not lie in this temperature range depending on the operation conditions of the engine. When the engine operates under a low-load condition, in particular, there occurs a case where the temperature of the exhaust gas does not rise and often fails to reach 250xc2x0 C., while during a high-load operation condition, too, there is a case where the exhaust gas temperature often exceeds the active temperature region and the PMs cannot often be burned continuously.
In the above cases, when according to the operation conditions, the temperature of the exhaust gas does not come into the active temperature region of the catalyst and the PMs trapped by the DPF do not burn, the PMs that do not burn remain trapped by the DPF and accumulate thereon. Then, when the engine operation condition changes and the temperature of the exhaust gas comes into the active temperature region of the catalyst, the PMs accumulated burn due to the action described above. At this moment, if the accumulated PMs start burning all at once, the temperature of combustion of PMs becomes as high as 2000xc2x0 C. causing such problems as melt-damage or the like to the filter body.
As described above, the continuous regeneration type DPF continuously conducts the regeneration when the exhaust gas is emitted while the PMs being trapped. Therefore, if the DPF can be continuously regenerated at all times whenever the engine is in operation, then, the temperature does not reach so high a temperature as to cause melt-damage. Because of this reason, it is important to maintain the temperature of the exhaust gas to lie within the active temperature region of the catalyst at all times.
By taking the above problems into consideration, the present applicant has proposed Japanese Patent Applications Nos. 2000-185897 and 2001-79266 in an attempt to maintain the temperature of the exhaust gas in the active temperature region of the catalyst.
The inventions of the above two applications were proposed based on a knowledge that the temperature of the exhaust gas is greatly affected by the amount of the air taken in the cylinder of the engine, and it becomes high as the excess air ratio (xcex), which it the ratio of the excess air to the fuel, approaches xcex=1 from a large state and as the temperature of the air taken in the cylinder becomes high (the temperature of the exhaust gas becomes low if they are reversed).
That is, Japanese Patent Application No. 2000-185897 discloses that there are arranged an intake throttle valve and a variable supercharger as air amount adjusting means, and the temperature of the exhaust gas is controlled by squeezing the intake throttle valve to decrease the amount of the intake air and by controlling the variable supercharger to increase the amount of the intake air.
Further, Japanese Patent Application No. 2001-79266 discloses that there are provided a so-called EGR passage communicating the exhaust gas passage of the engine with the intake air passage and an EGR valve for controlling the passage area of the EGR passage, and the temperature of the exhaust gas is controlled by controlling the flow rate of the EGR gas refluxed from the exhaust gas reflux (EGR) passage to the intake air passage side and by further controlling the amount of the intake air by the intake air shutter in the intake air passage and by the exhaust gas shutter in the exhaust gas passage.
In a diesel engine, the amount of the intake air is not usually controlled, and the excess air ratio (xcex) becomes large as the load is low, i.e., when the fuel is injected in small amounts, and becomes xcex=10 or larger during the idling operation. To bring the excess air ratio (xcex) close to 1, therefore, the amount of the intake air must be squeezed to a considerable degree.
Meanwhile, as is well known, the diesel engine is a combustion system based on the self-ignition by the compression. That is, the intake air is compressed based on the compression ratio of the cylinder defined by a bore of the cylinder and the stroke of the piston, the fuel is injected into the cylinder in which the temperature has been raised due to the compression and hence, the temperature of the fuel itself is elevated, so that the fuel is self-ignited while being vaporized. Therefore, the above method of decreasing the amount of the intake air hinders the elevation of the temperature of the intake air by the compression and produces a condition where the self-ignition does not easily take place. Namely, the above method arouses a new problem inducing incomplete combustion in the cylinder and emitting unburned HC (hydrocarbons).
FIG. 9 is a graph illustrating the effect produced as a result of squeezing the intake air shutter, in which the ordinate represents the exhaust gas temperature (xc2x0 C.) and HC (xc3x97100 ppm) and the abscissa represents the amount of the intake air [kg/h]. The amount of the intake air is limited by squeezing the intake air shutter stepwise. Solid lines in the graph represent a change in the exhaust gas temperature and the HC contained in the exhaust gas in the prior art, that were measured by operating the intake air shutter only. The data of FIG. 9 were measured by using a 4-cylinder 3-liter diesel engine while driving it at a rotational speed (Ne) of 1000 rpm, injecting the fuel at a rate (Q) of 10 mm3/st and measuring the exhaust gas temperature at the outlet of the exhaust manifold.
As will be obvious from FIG. 9, the exhaust gas temperature is about 150xc2x0 C. when the intake air shutter is not squeezed. The exhaust gas temperature gradually increases as the intake air shutter is gradually squeezed. However, the HC sharply increases as the amount of the intake air approaches 80 kg/h. In a region where the amount of the intake air lies between 70 and 80 kg/h, there takes place an abnormal combustion permitting HC to be emitted in amounts of exceeding 10,000 ppm. Then, the exhaust gas is spewed out as white smoke from the exhaust pipe, the combustion is no longer normally conducted, and the exhaust gas temperature is not raised any more no matter how the intake air shutter is squeezed. Therefore, the intake air shutter alone is not enough for raising the exhaust gas temperature to a sufficient degree.
The present invention has been accomplished in view of the above-mentioned points, and its principal technical assignment is to provide an improved device for purifying exhaust gas of diesel engines mounting a continuous regeneration type DPF by raising the exhaust gas temperature even under operation conditions where the exhaust gas temperature is low without deteriorating the fuel efficiency and by promoting the continuous combustion of PMs in the continuous regeneration type DPF, so that the PMs will not be deposited on the DPF.
In order to solve the above-mentioned technical assignment, according to the present invention, there is provided a device for purifying exhaust gas of a diesel engine equipped with a continuous regeneration type diesel particulate filter arranged in the exhaust gas passage of the engine, comprising:
an intake air shutter arranged in the intake air passage of the engine;
an exhaust gas temperature region detecting means for detecting an exhaust gas temperature region of the engine;
a control means for squeezing the opening degree of said intake air shutter when the exhaust gas temperature region of the engine, detected by said exhaust gas temperature region detecting means, is lower than a predetermined temperature region; and
an exhaust valve-operating mechanism equipped with an exhaust gas introduction mechanism for opening the exhaust valve of the engine for a short period of time during the intake stroke.
In order to solve the above-mentioned technical assignment according to the present invention, there is further provided a device for purifying exhaust gas of a diesel engine equipped with a continuous regeneration type diesel particulate filter arranged in the exhaust gas passage of the engine, comprising:
an intake air shutter arranged in the intake air passage of the engine;
an exhaust gas introduction mechanism for opening the exhaust gas passage to the cylinder during the intake stroke;
an exhaust gas temperature region detecting means for detecting the exhaust gas temperature region of the engine; and
a control means for squeezing the opening degree of the intake air shutter and for operating said exhaust gas introduction mechanism when the exhaust gas temperature region of the engine, detected by said exhaust gas temperature region detecting means, is lower than a predetermined temperature region.
It is desired that when the exhaust gas temperature region of the engine detected by the exhaust gas temperature region detecting means is lower than a predetermined temperature region, said control means controls stepwise so as to decrease the opening degree of said intake air shutter as the temperature region becomes low.
It is further desired that an exhaust gas shutter arranged in the exhaust gas passage and for squeezing the exhaust gas passage is provided, and the control means controls so as to squeeze the opening degree of the exhaust gas shutter when the exhaust gas temperature region detected by the exhaust gas temperature region detecting means is in a low temperature region. The opening degree of the exhaust gas shutter is controlled stepwise to be decreased as the temperature region becomes low.